The present invention relates to conveyor rolls used in high temperature applications, and more particularly concerns an improved end cap for such rolls.
A conveyor roll used in high temperature applications may comprise a ceramic spool having metal end caps. Typically, the ceramic spool comprises fused silica. The end caps permit facile mounting to a bearing or drive wheel. For example, in glass tempering applications, the ceramic spool supports glass sheet, and the end caps permit mechanical coupling to the drive mechanism. The end caps should securely adhere to the spool, thereby permitting the spool to rotate at the desired speed. Eccentric rotation is generally undesirable, as this would create an uneven support surface for the glass sheet.
The different thermal expansions of the ceramic spool and the metal end caps makes securely fastening the end caps to the spool difficult and can create eccentric rotations. Various methods have been proposed to overcome this difficulty. U.S. Pat. No. 3,867,748 teaches fastening end caps to a spool using an adhesive. U.S. Pat. No. 4,242,782 proposes fastening end caps using elastic O-rings. Adhesives and O-rings can become pliable and lose holding power at elevated temperatures causing eccentric rotation of the spool and slippage between the end caps and the spool.
Metallic connectors have also been used to secure an end cap to a spool. U.S. Pat. No. 5,316,129 describes the use of a helically wound coil between the spool and the end cap. The coil includes bent portions and straight portions that permit continuous contact between the end cap and the spool despite disparate thermal expansion coefficients. U.S. Pat. Nos. 5,906,567 and 5,370,596 describe curved bimetallic shims for securing the end cap to the spool. The curvature of the bimetallic shims changes with temperature thereby retaining a secure attachment between the spool and the end cap. Assembling and repairing a coil spring or bimetallic system can, however, be difficult.
Mechanical fasteners can be used to fasten an end cap to a spool. U.S. Pat. No. 4,751,776 shows an end cap having an annular distribution of screws that thread into a ferrule on the end of a spool. The screws are tightened to secure the end cap to the spool, but thermal expansion and contraction can loosen the screws causing eccentric rotation and slippage. Screws have also been keyed to a flat surface machined on face of the spool. FR 2 550 172 describes an end cap including a metallic collar between the spool and the end cap. The end cap is locked in place with a key fitted to a flat surface of the spool. The collar has a thermal expansion greater than the end cap, and is intended to compensate for the difference in thermal expansion between the end cap and the spool. In practice, the collar does not expand sufficiently and loosening of the end cap results. U.S. Pat. No. 5,146,675 discloses a screw that can be tightened to force a metal plate against a flat on the spool, which presses the spool against the inner surface of the end cap. The end cap includes an access opening that permits the screw, metal plate and end cap to be welded together, thereby preventing the screw from backing out. Effectively, the end cap and the spool connect across a single axis, that is, the screw. Thermal cycling across a single axis of contact can cause eccentric rotation and movement of the metal plate relative to the spool. Such movement can even cause the plate to contact the edge of the flat and crack the spool.
A need persists for an end cap that fixedly and centrally secures to a ceramic spool and is resistant to loosening after repeated thermal cycling. The end cap should also be easy to install and repair.
The objective of the present invention is to provide an end cap that fixedly attaches to a ceramic spool despite repeated thermal cycling. The cap includes a locking plate adapted to fit into tight connection with a groove in the spool by tightening of at least two pins, which fix the locking plate in place and prevent wobble. The pins are then prevented from loosening by welding in place.
One aspect of the invention shows an end cap comprising a ferrule and a locking plate. The locking plate is adapted to fit into a mated groove in the spool, and the ferrule is placed over the end of the spool. At least two pins extend through the ferrule and fix the locking plate in place. The pins may be screws or other similar mechanical fasteners. The pins are then welded to the end cap to prevent loosening.
A further aspect of the invention describes the pins as radially offset by about 15xc2x0-90xc2x0 and preferably around 60xc2x0. The offset creates two axes securing the locking plate and reducing its wobble, thereby reducing loosening and eccentric rotation of the spool.
Preferably, the pins are at or below the outer surface of the end cap. This permits the holes and the pins to be welded closed, which promotes a more aesthetic appearance, prevents build-up in the holes and loosening of the pins. Welding of the pins to the end cap is preferably accomplished using heat, but can also be effected by chemical action, such as with a high temperature adhesive.
In one embodiment, the end cap is tapped to receive the screws and the locking plate has two open holes for accepting the screws. Tightening the screws will drive the screws through the open holes. The screws, groove and ferrule fix the locking plate in place and secure the end cap to the spool.
In a second embodiment, screws driven into blind untapped holes in the locking plate press the spool against the inside surface of the end cap and frictionally improve the contact between the end cap and the spool. Alternatively, holes in the locking plate are tapped and holes in the ferrule are untapped. In either situation, the end cap is securely and frictionally mounted to the spool.
In another embodiment, a supplemental means of fixing the end cap to the spool is provided. Conveniently, an adhesive may be used, such as a room temperature vulcanizing (RTV) silicone, between the end cap and the spool.